1. Field of the Invention
This invention relates to a method for electrolyzing halides of monovalent alkali metals using an electrolytic diaphragm, and more specifically, to a method for electrolyzing halides of monovalent alkali metals using a cation exchange membrane composed of a graft copolymer of a polyolefin main chain to which a side chain composed mainly of a dihydroxystyrene is grafted.
2. Description of the Prior Art
In the method of electrolyzing an aqueous solution of an alkali metal halide in an electrolytic cell including an anode, a cathode, a diaphragm disposed between them for separating the electrolytic cell into an anode compartment and a cathode compartment, and means provided outside of the cell for passing an electric current between the anode and the cathode, the feeding of the aqueous solution of the alkali metal halide into the anode compartment and the subsequent passing of a current between both electrodes result in the conversion of the halogen ion to the halogen at the anode. The alkali metal ion moves to the cathode compartment via the diaphragm, and an alkali metal hydroxide and hydrogen gas are generated at the cathode. Previously, a porous diaphragm made of asbestos has frequently been used for this purpose. However, since an asbestos porous diaphragm does not possess selective permeability between positive and negative ions and between monovalent and polyvalent ions, a part of the hydroxide ion formed in the cathode compartment diffuses into the anode compartment through the diaphragm to cause a reduction in current efficiency. At the same time, very small amounts of divalent or higher cations such as iron, magnesium or calcium contained as impurities also move to the cathode compartment together with the alkali metal ion, and thus cannot be removed. Thus, in order to prevent diffusion of hydroxide ions into the anode compartment, a technique is employed of flowing a part of the anodic solution into the cathode compartment through the diaphragm. However, an enormous cost in the subsequent concentrating and purifying steps is required because the aqueous solution of the alkali metal hydroxide obtained in the cathode compartment contains a large quantity of the alkali metal halide and traces of polyvalent metal ions, and the concentration of the resulting alkali metal hydroxide cannot be increased.
It is known that the use of a cationic exchange membrane as the diaphragm obviates the above defect. If, a cation exchange membrane having an ideal selective permeability to monovalent cations is used as a diaphragm, alkali metal hydroxides in high concentrations can be obtained from the cathode compartment without involving the above difficulties because the cation exchange membrane does not permit the permeation of hydroxide ions, halogen ions and polyvalent metal ions such as iron or magnesium. However, it is very difficult in practice to produce membranes having an ideal permselectivity to monovalent cations. Conventional cation exchange membranes have proved to be not entirely feasible for one or more reasons. For example, these membranes cannot ensure sufficient current efficiency or sufficient purity or concentration of the product. Moreover, since the membranes are exposed to severe conditions, they do not have sufficient endurance for use for prolonged periods of time.